English for thermal engineering chapter 4 drying processes

In these applications, the drying process must be meticulously controlled to avoid damaging sensitive compounds, ensuring that the final product retains its potency... This balance is cr

Ho Chi Minh City, February of 2025

VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

-❁ ✺

❁ -ENGLISH FOR THERMAL ENGINEERING

CHAPTER 4: DRYING PROCESSES

Instructor Name: Nguyen The Bao Student Name: Thai Thanh Duy Student Code: 2113038 Class: L02

CONTENTS

4.2 Introduction to Drying 4

4.2.1 Advanced Exercise 4

4.3 Basic Principles of Drying 5

4.3.1 Advanced Exercise 5

4.4 Classification of Dryer 7

4.4.1 Advanced Exercise 7

4.5 Typical Industrical Dryer 9

4.5.1 Advanced Exercise 9

4.6 Drying Applications in Different Industries 12

4.6.1 Advanced Exercise 12

4.7 Drying Rate and Factors Influencing It 14

4.7.1 Advanced Exercise 14

4.8 Rotary Drum Dryer 16

4.8.1 Reading Comprehension 16

4.8.2 Grammar in Use 17

4.8.3 Advanced Exercise 17

4.9 Conveyor Dryers 20

4.9.1 Reading Comprehension 20

4.9.2 Gramma in Use 20

4.9.3 Advanced Exercise 21

4.10 Advanced Drying Techniques 21

4.10.1 Reading Comprehension 21

4.10.2 Grammar in Use 21

4.10.3 Advanced Exercise 22

4.11 Quiz 25

4.11.1 Multiple Choice Questions 25 4.11.2 Short Answer Questions 25

In the food industry, drying plays an integral role in extending the shelf life of perishable items Fruits, for example, are often freeze-dried to retain their original structure and nutrients, preserving both their flavor and appearance In contrast, air-drying methods, while effective, can result in a leathery texture, as seen in the production of raisins from grapes Meat products such as jerky benefit from drying as the reduced moisture content prevents microbial growth, ensuring safety and longevity Similarly, dairy products like spray-dried milk powder rely on carefully managed drying processes to maintain a stable, white appearance and a long shelf life while preventing issues like clumping or caking during storage

The pharmaceutical industry also depends on drying to produce high-quality, stable products Many powdered drugs, including antibiotics, are dried to prevent microbial contamination and chemical degradation Drying is equally important in the production of herbal extracts, where it helps preserve the active compounds necessary for therapeutic efficacy In these applications, the drying process must be meticulously controlled to avoid damaging sensitive compounds, ensuring that the final product retains its potency

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Beyond food and pharmaceuticals, drying is indispensable in other industries as well

In the textile industry, proper drying techniques prevent the growth of mold and preserve the durability of fabrics, ensuring that dyes and fibers remain intact The paper and pulp industry benefits from controlled drying to prevent warping and to enhance the strength and longevity of the final paper products In the chemical industry, drying is crucial for the stability of powdered chemicals, fertilizers, and pigments, where moisture could otherwise trigger unwanted chemical reactions or degradation

While drying extends the shelf life of products by reducing water activity and thus preventing spoilage, it also influences key quality attributes Excessive heat during drying can lead to the loss of volatile compounds, which are essential for the flavor and aroma in many foods For instance, the rich aroma of coffee beans can be compromised if not dried correctly Nutrient degradation is another concern; sensitive nutrients like vitamin C can deteriorate when exposed to improper drying conditions Therefore, techniques such as freeze-drying are often favored for products where the retention of nutritional and sensory qualities is paramount

In conclusion, drying is a vital preservation technique that enhances the stability and shelf life of products in multiple industries Whether it is the preservation of food items, the stabilization of pharmaceutical compounds, or the maintenance of quality in textiles, paper, and chemicals, drying helps prevent microbial, enzymatic, and chemical spoilage The challenge lies in choosing the appropriate drying method to ensure that moisture is removed without sacrificing the product’s inherent quality By striking this balance, industries can deliver products that are both safe and of high quality, ultimately benefiting consumers and manufacturers alike

4.3 Basic Principles of Drying

4.3.1 Advanced Exercise

Analyze the drying process of a specific material Discuss how temperature, humidity, and airflow affect the drying rate and product quality

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Drying is an age-old preservation method that extends the shelf life of perishable products while maintaining their nutritional value and flavor One material that benefits significantly from this process is the apple The drying process of apples is influenced by several key factors, including temperature, humidity, and airflow Each of these factors plays a vital role in determining the drying rate and the quality of the final product By understanding and balancing these elements, manufacturers can optimize the drying process to produce apples that are not only safe for consumption but also retain much of their original appeal

Temperature is perhaps the most critical variable in the drying process It directly affects the rate of moisture evaporation from apple slices At low temperatures—below 50°C (122°F)—the drying process is slow While this reduces the risk of overheating and nutrient degradation, prolonged exposure to such conditions may promote enzymatic browning and microbial growth if not carefully controlled In contrast, a moderate temperature range between 50°C and 70°C (122°F–158°F) is often optimal This range facilitates efficient moisture removal while preserving the texture, flavor, and nutritional content of the apples However, if the temperature exceeds 70°C (158°F), the risk of case hardening increases In such instances, the surface of the apple dries out too quickly, forming a hard layer that traps moisture inside, potentially compromising the texture and promoting microbial issues

Humidity, or the moisture content in the surrounding air, is another crucial factor that influences the drying process Drying occurs when there is a significant moisture gradient between the apple slices and the ambient air High humidity levels—typically above 60%—impede moisture evaporation by reducing this gradient, thereby prolonging the drying process and potentially leading to spoilage or microbial contamination Conversely, very low humidity levels (below 30%) can accelerate drying but may also result in overly brittle apples that lack the desired juiciness and texture The ideal scenario is a controlled humidity environment, usually in the range of 30% to 50%, which balances efficient drying

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with the maintenance of quality This controlled setting ensures that the apples are dried uniformly without compromising their flavor or nutritional content

Airflow is the third critical factor in the drying process It aids in the efficient removal

of moisture by constantly replacing humid air with drier air When airflow is too low, moisture can accumulate around the apple slices, slowing down the drying process and potentially increasing the risk of microbial growth On the other hand, excessive airflow can lead to rapid drying of the apple’s surface, once again resulting in case hardening and uneven moisture distribution The optimal airflow is one that is both steady and controlled, ensuring that moisture is continuously removed from the surface while allowing the interior moisture to migrate gradually to the surface for evaporation This balance is crucial in achieving a uniform drying process that preserves the overall quality of the apple

In conclusion, the drying process of apples is a delicate interplay of temperature, humidity, and airflow Each factor must be carefully managed to ensure that moisture is removed efficiently while maintaining the apple’s texture, flavor, and nutritional value A moderate temperature range of 50°C–70°C, a controlled humidity level between 30% and 50%, and a balanced, steady airflow are essential for producing high-quality dried apples

By fine-tuning these variables, producers can optimize the drying process, resulting in a final product that is both safe and appealing to consumers

of hot air ensures that moisture is rapidly carried away from the product, leading to faster drying times Moreover, a well-designed convection system can distribute heat uniformly, which helps in reducing the risk of localized overheating This method is also highly scalable, making it attractive for large-scale industrial operations Additionally, because the heat is delivered via a gaseous medium rather than direct contact, the risk of product contamination is minimized

However, convection drying is not without its disadvantages The process typically requires high energy consumption since large volumes of air or gas must be heated to the desired temperature This can increase operational costs significantly Furthermore, prolonged exposure to hot air can potentially degrade heat-sensitive materials, risking the loss of nutritional value, flavor, or even structural integrity The dynamic airflow may also generate airborne dust, necessitating additional measures for dust control and filtration

In contrast, conduction dryers utilize direct contact between the product and a heated surface to transfer heat This method is inherently more energy-efficient because the energy

is directly imparted to the product, reducing the need to heat large volumes of air Conduction drying allows for more controlled conditions; for instance, it is possible to operate at lower temperatures, which is particularly beneficial when drying delicate or heat-sensitive materials Another advantage is that the absence of high-velocity airflows minimizes the dispersal of particles, thereby reducing the issue of dust generation

Nonetheless, conduction drying has its own set of limitations One of the primary concerns is the risk of surface overheating, as direct contact with a heated surface can lead

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to product degradation, caramelization, or even burning if not properly controlled This method can also result in uneven drying, as some areas may absorb more heat than others, potentially leading to inconsistent moisture removal Furthermore, conduction dryers are generally less scalable compared to convection dryers, especially in batch processes, which may limit their application in high-volume industrial settings

When considering the drying of heat-sensitive materials, conduction drying emerges

as the more suitable option This is especially true when the process is combined with vacuum drying techniques Vacuum conduction dryers operate at lower temperatures by reducing the ambient pressure, which allows moisture to evaporate without exposing the material to excessive heat Such conditions are ideal for pharmaceuticals, biological materials, and certain food products, where maintaining the integrity of the product is paramount Although convection drying methods such as freeze-drying or low-temperature fluidized bed drying can also be adapted for heat-sensitive applications, they typically require a delicate balance between effective moisture removal and temperature control

In conclusion, both convection and conduction dryers have distinct advantages and drawbacks Convection dryers excel in efficiency and scalability but may compromise product quality through high energy consumption and potential overheating Conversely, conduction dryers offer energy efficiency and more controlled drying conditions, making them particularly well-suited for heat-sensitive materials despite challenges such as uneven drying and lower scalability The choice between these drying methods ultimately depends

on the specific requirements of the product, with conduction drying - especially under vacuum conditions - being the preferred method for preserving the quality of delicate, heat-sensitive materials

4.5 Typical Industrical Dryer

4.5.1 Advanced Exercise

Design a drying process using a rotary drum dryer for a specific bulk solid material Consider factors such as drum speed, air flow and temperature to optimize the drying rate

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Roraty drum dryers are widely utilized in industrial processes for the efficient drying

of bulk solid materials One specific application is the drying of silica sand, a critical material in industries such as construction, foundry operations, and glass manufacturing The design of an effective rotary drum drying process for sand involves the careful optimization of key parameters including drum dimensions and speed, airflow, and temperature By tuning these factors, engineers can achieve rapid moisture removal, ensure product quality, and maintain energy efficiency

Silica sand, with an initial moisture content of approximately 8–12%, must be reduced to a final moisture level of 0.5–1% to meet industry requirements The physical properties of sand—such as its bulk density, particle size distribution, and thermal characteristics—play a significant role in the drying process In this context, the rotary drum dryer is an ideal choice, as it allows for continuous operation and efficient handling

of materials The drum itself is typically designed with a diameter of around 2.5 meters and

a length of approximately 10 meters, which provides ample space for the material to be thoroughly processed during its passage through the dryer

A critical aspect of the design is the drum’s rotation speed, generally set between 4 and 6 revolutions per minute (RPM) This moderate speed ensures that the sand is tumbled evenly, promoting uniform exposure to the drying medium while preventing material buildup Furthermore, the drum is inclined slightly - typically at an angle of 3 to 5 degrees

to facilitate a continuous, gravity-assisted movement of the sand through the dryer This design consideration helps achieve a consistent retention time, usually between 15 and 25 minutes, which is sufficient to lower the moisture content without risking over-drying or causing excessive dust formation

Airflow and temperature are equally important in optimizing the drying process In a rotary drum dryer, a concurrent airflow configuration is often employed, meaning that both the hot air and the sand move in the same direction This arrangement is particularly advantageous because the wet, incoming sand is exposed to the hottest air, which

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maximizes the initial drying rate while protecting the drier product at the outlet from excessive thermal exposure For this application, the inlet air temperature is typically maintained between 300 and 400°C, while the exhaust air is cooled to a range of 100 to 150°C Such temperature settings not only accelerate moisture evaporation but also ensure that the sand does not suffer from thermal degradation

Airflow rates, commonly ranging from 5,000 to 7,000 m³/h, are carefully controlled using adjustable dampers These dampers allow operators to fine-tune the amount of air passing through the dryer, ensuring that moisture-laden air is efficiently replaced with drier air, thus maintaining a consistent drying environment This controlled airflow is crucial for preventing localized overheating and ensuring that the moisture removal rate remains steady throughout the process

The heat source for the rotary drum dryer is typically a direct-fired system using natural gas or biomass burners Direct heating facilitates rapid energy transfer to the sand,

an essential factor when operating at high temperatures required for fast drying Proper insulation of the drum further enhances energy efficiency by minimizing heat loss, which

is important given that energy consumption can reach between 1,200 and 1,500 kcal per kilogram of evaporated water

In addition to the primary design parameters, several ancillary components are integrated into the system to ensure operational stability and product quality Temperature sensors, such as thermocouples, are strategically placed at the inlet, mid-section, and outlet

of the dryer These sensors provide real-time data that allows for continuous monitoring and fine adjustments to the process Moreover, a cyclone separator coupled with a baghouse filter is used to control dust emissions, ensuring that the working environment remains clean and that the dried sand meets stringent quality standards

In summary, the design of a rotary drum drying process for silica sand requires a holistic approach that integrates material characteristics with precise control over drum speed, airflow, and temperature By utilizing a drum with optimal dimensions and a

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controlled rotation speed, and by ensuring that the airflow and temperature are maintained within specific ranges, the process achieves efficient moisture removal while preserving the quality of the sand This careful balance of factors not only improves the drying rate but also enhances energy efficiency, making the rotary drum dryer an indispensable tool in industrial applications where the production of high-quality dried sand is essential

4.6 Drying Applications in Different Industries

4.6.1 Advanced Exercise

Discuss the specific drying methods used in the chemical industry for drying catalysts and polymers Explain the importance of drying in ensuring product quality and performance

Drying is a critical step in the chemical industry, particularly when it comes to processing catalysts and polymers This process is not merely about removing moisture; it

is integral to ensuring that the final products exhibit the desired quality, efficiency, and performance characteristics In the manufacturing of catalysts and polymers, moisture can significantly impair functionality—by clogging active sites in catalysts or interfering with polymerization reactions—making the selection of an appropriate drying method essential For catalysts, which are typically produced as fine powders or in slurry form, the drying process must be conducted with utmost care Catalysts rely on high surface area and porosity to facilitate chemical reactions Any residual moisture can block these vital active sites, reduce the effective surface area, or even cause particle agglomeration, ultimately diminishing catalytic performance One common method used for drying catalysts is vacuum drying By operating under reduced pressure, the boiling point of water decreases, allowing for moisture removal at lower temperatures This gentle approach minimizes thermal stress and preserves the delicate structure of the catalyst Fluidized bed drying is another preferred method, wherein catalyst particles are suspended in a stream of hot air or inert gas This suspension ensures uniform heat and mass transfer throughout the material, maintaining consistent drying while reducing the risk of agglomeration In cases where

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catalysts are produced from liquid slurries, spray drying is often employed The process involves atomizing the slurry into fine droplets, which rapidly lose their solvent as they pass through a heated chamber This method not only speeds up the drying process but also allows for precise control over particle size and morphology For catalysts that are exceptionally sensitive to heat or chemical changes, freeze-drying, or lyophilization, offers

an alternative despite its higher cost and longer processing times By freezing the catalyst slurry and then sublimating the ice under vacuum, freeze-drying preserves the structural integrity and activity of the catalyst without exposing it to harsh thermal conditions Polymers, another vital group of materials in the chemical industry, also demand rigorous drying protocols Residual moisture or solvents in polymers can lead to undesirable side effects such as hydrolysis, defects during processing, and compromised mechanical properties in the final product One effective method for drying polymers is the use of vacuum ovens The vacuum lowers the boiling point of water and other volatile components, permitting drying at reduced temperatures and thus protecting the polymer from thermal degradation Fluidized bed drying is also applicable for polymers, especially

in the form of powders or granules This method ensures uniform drying by constantly circulating the particles in a stream of air, preventing the formation of moisture pockets and maintaining consistent physical properties throughout the batch Infrared drying is another technique particularly suited to polymer films and coatings Infrared energy provides rapid and controlled heating, effectively removing moisture without penetrating too deeply into the material, thereby preserving the integrity of delicate polymer structures Additionally, spray drying is utilized for polymer powders where a fine and uniform particle size is desired The rapid evaporation process inherent in spray drying minimizes the likelihood of particle agglomeration and helps produce polymers with controlled morphology and improved flow characteristics

The importance of these drying methods in ensuring product quality and performance cannot be overstated In catalysts, for instance, inadequate drying can lead to blocked active sites and reduced reaction efficiency, which directly impacts the catalyst's effectiveness in